This invention relates to an improved resin-containing coating solution which, when applied as a solvent-based resin solution to a fabric substrate and processed to coagulate the resin, results in a waterproof, microporous, moisture vapor permeable fabric. In particular, this invention relates to a thickener system for such resin-containing coating solutions.
Waterproof, moisture-permeable coated fabrics with good hand for use in clothing such as raincoats, work clothes, tents, canvas shoes, and raingear, made by a wet coagulation method, are described in U.S. Pat. No. 4,429,000. In that method, a water-miscible polar organic solvent solution of a resin such as polyurethane is applied to a base fabric which is then immersed in a water bath to coagulate the resin from the solution and deposit a thin, microporous, waterproof yet water vapor permeable layer on the fabric. Typical coating solutions contain a resin, usually a polyurethane elastomer, and optionally a water repellent agent, a thickener, a surfactant and possibly other adjuvants, all dissolved in a water-miscible polar organic solvent such as dimethylformamide, N-Methyl-2-Pyrrolidone dimethylacetamide or dimethylsulfoxide. The coagulating bath contains water and with up to 20% by weight of the same or a compatible polar solvent.
Coating viscosity must be carefully controlled to adjust penetration and interstitial strike-through, especially on loosely-woven and textured fabrics.
When coating textile fabric with a resinous material dissolved in a liquid medium, final distribution of the dried coating depends on several factors, including the coating method, fabric geometry, and coating viscosity. In many situations, coating viscosity is the variable most easily manipulated to control coating distribution. When dealing with true solution coatings, the solute molecular weight, solute concentration, and nature of the solvent have an influence on coating viscosity. However, optimum physical and handling properties of the coating are often obtained within specific ranges of molecular weight and polymer concentration. Application techniques can also severely limit solvent choices. With these practical operational constraints, a viscosity control additive is required.
Traditional thickeners for such resin-containing solutions are natural and modified gums, solvent-interactive fillers, and high molecular weight synthetic polymers. The choice of thickening agent has traditionally been made on the basis of compatibility, coating performance requirements and economics.
It is an object of this invention to provide a convenient, reliable thickener system providing the required ease of processing without distracting from the desired physical properties of the finished product. Water-coagulable coatings for textiles must exhibit coagulation rates within relatively narrow limits to maintain reproducible fabric properties. Candidate thickeners must have little or no effect on coagulation rates. Disclosed is a thickener system that satisfies these various requirements.